1. Field of the Invention
The present invention relates to an active-matrix addressing Liquid-Crystal Display (LCD) device having a so-called active-matrix substrate on which pixel electrodes and Thin-Film Transistors (TFTs) are arranged in a matrix array.
More particularly, the invention relates to an active-matrix addressing LCD device with an active-matrix substrate on which multilayer-structured conductive lines are formed along with pixel electrodes and TFTs, which suppresses effectively aluminum (Al) hillocks without any complicated conductive line structure to thereby decreasing the connection resistance increase of the lines due to heat or moisture and improving their connection reliability.
2. Description of the Related Art
The active-matrix addressing LCD device has a typical configuration as follows.
The LCD device of this type comprises an active matrix substrate and an opposite substrate coupled to each other in parallel to form a specific gap between them with a sealing member. The gap between the substrates forms a closed space for confining a specific liquid crystal. Thus, the space (and the liquid crystal) is sandwiched by the substrates.
Pixel areas are arranged in a matrix array on the active matrix substrate. TFTs are arranged on the active matrix substrate to correspond to the respective pixel areas, which are to control the voltages applied to the corresponding pixel electrodes. Opposing electrodes are arranged on the opposite substrate. Specific voltages are applied across the electrodes arranged on these two substrates to drive the liquid crystal, thereby displaying images on the screen of the LCD device.
If the LCD device is of the vertical electric-field type where electric fields are generated to be approximately vertical to the substrates in the closed space (i.e., in the liquid crystal) on operation, the active matrix substrate comprises a transparent glass plate. Scan lines extending in the first direction are arranged at equal intervals on the surface of the glass plate in the second direction, where the second direction is perpendicular to the first direction. Signal lines extending in the second direction are arranged at equal intervals on the surface of the glass plate in the first direction. The pixel electrodes are arranged at the respective pixel areas defined by the scan lines and the signal lines thus intersected. The TFTs are arranged in the respective pixel areas, The gate electrodes, the drain electrodes, and the source electrodes of the TFTs are connected to the scan lines, the signal lines, and the pixel electrodes, respectively.
Accordingly, when specific electric currents are supplied to one of the scan lines and one of the signal lines, respectively, the TFT located at the intersection of these scan and signal lines are turned on, allowing a specific voltage to be applied to the relating pixel electrode to the TFT in question. This operation is conducted for all the necessary pixels. Thus, a desired image is displayed on the screen of the device
Each of the scan lines has a scan-line terminal at its end. Each of the signal lines has a signal-line terminal at its end. These scan- and signal-line terminals are used to connect a tape-shaped cable for interconnecting the scan and signal lines with a specific driver circuit unit. The cable includes a set of conductive or wiring lines previously connected to the driver circuit unit. Thus, the scan and signal lines are connected to the unit by way of the corresponding lines of the cable.
With the active-matrix substrate of this type, there is the need to decrease the size of the pixel electrodes and to decrease the electrical resistance of the scan and signal lines themselves by using any other conductive material of lower electrical resistance and any other structure. This is responsive to the recent requirement of enlarging the LCD device and of raising the density of the elements or components used in the device.
Moreover, since the scan and signal lines need to be connected to the conductive lines of the tape-shaped flat cable at their terminals, these terminals need to be made of a reliable material or materials to prevent the connection reliability at the terminals from degrading due to invasion of moisture.
To meet the above-described need, various improvements have been made and disclosed so far.
For example, the Japanese Non-Examined Patent Publication No. 7-120789, which was published in 1995, discloses a multi-level conductive structure applicable to the scan and signal lines of the LCD device This structure includes a lower aluminum (Al) film and an upper titanium nitride (TiN) film. The Al film is used to lower the electrical resistance of the structure or the lines. The TiN film is used to prevent the underlying Al film from being exposed to various chemicals during the fabrication processes and therefore, corrosion of the TiN firm can be avoided. This means that a highly reliable connection structure is possible.
However, the multi-level conductive structure disclosed by the Publication No. 7-120789 has a problem that hillocks tend to occur on the Al film. As known well, “Al hillocks” are small hills or protrusions formed on the surface of the Al films which are caused by the fact that a compressive stress is applied to the Al film in a heat treatment process and then, the stress is relaxed or decreased with time to thereby diffuse the Al atoms of the film outwardly. The Al hillocks will cause various defects (e.g., inter-level short-circuit) and as a result, the fabrication yield degradation will be more likely.
A technique to prevent the Al hillocks is disclosed by, for example, the Japanese Non-Examined Patent Publication No. 7-58110 published in 1995. This technique includes a multilevel conductive structure with TiN, Ti, Al, TiON, and Ti films. In other words, this technique includes a TiN/Ti/Al/TiON/Ti structure. The top-level TiN film is used to prevent reflection of light and to ensure desired etch selectivity in the contact-hole formation process. The upper Ti film is to decrease the electrical connection resistance. The middle-level Al film is used as a conductor material. The TiON film is used as a diffusion barrier film against silicon (Si) The bottom-level or lower Ti film is to decrease the electrical connection resistance.
With the technique disclosed by the Publication No. 7-58110, the Al film is sandwiched by the top-level TiN film and the underlying TiON film to prevent the Al hillocks (and alloy pits). However, the Publication No. 7-58110 discloses only the TiN/Ti/Al/TiON/Ti structure and fails to disclose other conductive or wiring structures that are effective to prevent the Al hillocks. Therefore, it is not clear that whether or not the TiN/Ti/Al/TiON/Ti structure disclosed therein is effective or advantageous to the case where the conductive or wiring structure is not contacted with a semiconductor layer. For example, if the disclosed structure is applied to the gate electrodes of the TFTs of the LCD device described above, the gate electrodes are not contacted with a semiconductor layer but a dielectric glass plate. In this case, it is not clear whether or not the disclosed structure is effective or advantageous.
In particular, the TiON film of the disclosed structure, which is used as the diffusion barrier film against Si, is unnecessary for the LCD device, because the gate electrodes are not contacted with a semiconductor layer. The TiON film only makes the structure complicated.
Furthermore, according to the information disclosed by the Publication No. 7-58110, it is seen that the combination of the TiN film and the Ti film located above the Al film and the combination of the TiON film and the Ti film located below the Al film are effective to the cases disclosed therein. However, it is not clear whether or not the disclosed advantages in the Publication No. 7-58110 are expected as well even if some of the constituent films of the TiN/Ti/Al/TiON/Ti structure is/are cancelled.